1. Field of the Invention
The invention is in the field of infra-red cryogenically cooled detectors, used to image thermal objects disposed in certain field of view. In special, the current application deals with changing detected spectral range by actions done outside cold shield of the detector.
2. Description of Related Art
An infra-red cryogenically cooled detecting device 17 of the prior art is illustrated in FIG. 1. An infra-red detector 5 is disposed on a cold finger 30, and placed within a cold shield 1. A cold filter 14 is thermally coupled to cold shield 1, and filters the radiation outside the detecting range of detector 5 to prevent undesired heating of the detector. Cold filter 14 may have even a smaller spectral range as needed by a specific task.
Cold shield 1 is located within a vacuum cell or Dewar Detector Cooler (hereafter DDC) 9. The incoming radiation from a certain field of view (FOV), which includes the object desired for imaging, enters vacuum cell 9 through vacuum tight window 32.
Sometimes, there is a demand to operate in two different spectral ranges. Implementation of this demand entails difficulty because the spectral range is determined by filter 14 which is cooled by rigid attachment, by welding for example, to cold shield 1 situated within DDC 9. Dewar 9 itself is drawn to high vacuum to reduce heat flow. It is also problematic to employ moving parts within the vacuum due to generation of dust and problems of lubrication. It is therefore not simple to exchange the filter within vacuum cell 9.
According to the conventional approach, it is assumed that any location of the spectral filter outside Dewar 9 would lead to problematic noise and degradation of the images due to thermal emissions from the filter itself and/or thermal energy from the environment incident on the detector side of the filter and reflected onto a detector or a detector array 5.
The present invention describes several devices and corresponding methods to operate a thermal imaging system switchably in several spectral ranges using a single cooled detector, whereas switching between spectral ranges is done completely outside the Dewar with no particular difficulty, and without significant loss of sensitivity or significant addition of undesired radiation.